During the fabrication of electronic packages, an integrated circuit chip is mounted on to a ceramic carrier, a metallic cap crimped onto the ceramic carrier over the chip and ceramic carrier is soldered to a printed circuit board. The metallic cap protects the circuitry on the integrated circuit chip and the ceramic carrier. However, crimping the cap does not provide a complete seal, and it is necessary to utilize another means of protecting the chip from subsequent processing, where harsh cleaning solvent chemicals are used, as well as from moisture in the environment. This can be accomplished by sealing the "back" of the ceramic carrier, the surface which is soldered to the printed circuit board, with a silicone or epoxy based coating composition. Several requirements exist for the coating material used in the backseal process. The material must have a low coefficient of thermal expansion (CTE), to match the CTE of the ceramic substrate. It must have excellent solvent resistance to withstand the chemicals in later processing. The composition should be 100% reactive, i.e., no volatile solvents are present after the curing process. The coating material must also be physically and thermally durable to survive the bubble leak and wave solder tests used to eliminate defective modules prier to card assembly. Finally, the formulation must be relatively inexpensive, as large volumes of the material are required for the backseal process.
In the past, an anhydride cured, bisphenol A (4,4'-isopropylidenediphenol) based epoxy, a proprietary product by 3M named Scotchcast, (Scotchcast is a register trademark of Minnesota Mining and Manufacturing Company) was employed as the backseal material. However, several disadvantages exist with Scotchcast: primarily, a limited-pot life and lengthy cure schedules, but also, problems with incomplete reaction of the anhydride. Scotchcast is a two-part system consisting of an epoxy and an anhydride hardener in a polyether polyester flexiblizer medium. Once the two parts are mixed, the mixture must be refrigerated due to the room temperature reactivity of the anhydride. Even with refrigeration, the pot life of the mixture is approximately five hours at 40.degree. C., less than a typical eight hour shift on the semiconductor line. In addition, curing times in excess of eight hours are routine. The anhydride used in Scotchcast is cis-hexahydro-phtalic anhydride (HHPA) which is solid at room temperature, and therefore, HHPA has a tendency to crystallize out of the mixed material. Furthermore, HHPA is hydroscopic, resulting in the generation of free acid which can catalyze an etherfication reaction. The optimum HHPA concentration of 75-85 parts per 100 resin (phr) approaches a one to one stoichiometry which depresses the overall end use material properties obtainable with the epoxy material. Decreased solvent resistance and decreased thermal stability occur at the 1:1 stoichiometry. Lower concentrations of the anhydride decrease the heat distortion temperature, increase weight loss and impart poor solvent resistance. High concentrations of the HHPA promote the etherfication, resulting in residual unreacted anhydride. The slow curing reaction typically requires extended bakes at temperatures high enough to insure the adequate thermal and physical resistance through subsequent processing. If the HHPA is either unreacted or has crystallized out, the anhydride can break down thermally during the curing process or wave solder tests into volatile solvents. The volatile solvents evaporate, and thereby cause pin holes or cracking in the backseal. The pin holes or cracking destroy the integrity of the module, and allow cleaning solvents or moisture access to the chip circuitry.
Because of the deficiencies of the Scotchcast material for the backseal process, an effort began to replace the material. For the new material, it would be desirable to have a coating composition, which in addition to meeting the requirements above, also is a one part, room temperature stable epoxy system possessing a much more rapid curing process to reduce cycle time. Ideally, the one-part epoxy system would have an indefinite pot life. Further, in view of the problems associated with the HHPA anhydride, it would be advantageous if the new material utilized a different curing method.
The requirement for a one-part epoxy coating composition can be met by the use of a photocopolymerizable composition. It is well known that mixtures of epoxies and hydroxyl containing materials can be radiation cured by the use of onium salts. The onium salts form stable formulations with epoxies and hydroxyl containing materials in an environment free of ultraviolet light radiation. When onium salts are radiated with ultraviolet light, a cationic catalyst composition is formed which cures the mixture. It is also known that the particular hydroxyl containing material selected as an ingredient in the radiation curable epoxy affects the cure rate and the properties of the cured material.
While there are prior art photocurable epoxy systems which are one part, room temperature stable compositions possessing a more rapid curing cycle than the Scotchcast material, they lack the physical properties which make Scotchcast attractive. After exhaustive experimentation, none of these existing photocurable epoxies tested possessed the required properties of a backseal material: low CTE, excellent solvent resistance and physical and thermal durability. Further, a problem with the use of a purely photocurable composition is that not all of the backseal material is accessible by light, particularly the material behind the crimp in the cap. Therefore, a purely photocurable composition would pose unacceptable risks to leave uncured epoxy in the cap.